1. Field of the Invention
This invention relates to a method of preparing composite aluminum material to be formed integrally by cast bonding aluminum or aluminum-alloy to an aluminum member, which comprises aluminum material.
2. Description of the Prior Art
Aluminum material or aluminum alloy material (hereinafter referred to as aluminum material) has been used not only in airplanes but also in various kinds of parts, due to its light weight and strength. Recently, there has been a demand to have the aluminum material provided with a specified property, such as high corrosion resistance, on all or part of its surface without losing the advantage of light weight. Thus light metal materials are formed by forming an aluminum layer having high corrosion resistance on an aluminum alloy casting having a high mechanical strength and by providing roller aluminum plate having a high mechanical strength on the aluminum alloy castings.
In the case of providing a specified property on all surfaces of an aluminum casting or partial surface thereof or in the case of providing a special shape on one part of an aluminum casting for a specified function, it might be considered to make use of the internal chill method at the time of casting molten aluminum on the aluminum material member which is prepared in advance. This method has been used often in the case of iron (ferrous) material. However, in the case of aluminum material, there is a disadvantage in that the surface of the member is covered with a fine oxide film, so that the aluminum layer, which is formed from molten aluminum, cannot bond sufficiently to the surface of the member. For affecting sufficient bonding of the layer to the member, attempts have been made to raise sufficiently the temperature of the molten aluminum or pre-heat the member in advance to result in good bonding. However, the conditions required to achieve sufficient bonding are quite critical with small ranges, and it is difficult to get even bonding.
Accordingly, this internal chill method is effective only for the composite material which needs to be formed by being covered with the aluminum, but not for the composite material which may be exposed to large temperature changes or mechanical force, or which needs high mechanical strength or air-tightness.
Furthermore, it is also considered a use of the internal chill method when applying flux on the surface of the member, but the conventional flux comprising ZnCl.sub.2 or NaCl, etc., does not have sufficient ability to remove oxide film. In addition, in case of a thin layer thereof, the flux may not be effective for that purpose and, to the contrary, a portion of the flux may remain as a thick layer in the composite material, thereby preventing a high bonding strength. Additionally a residue of the chloride on the surface of the member may corrode the composite material later formed.
As a non-corrosive flux, potassium tetrafluoroaluminate (KAlF.sub.4) has been known, but this is insoluble in solvents such as water, so that when using, it needs to be powdered and even application is not effected. Furthermore, even though it is applied in suspension, at the time of setting the member into a mold after drying, the flux releases from the surface to have been coated.